This invention relates to a pressure swing adsorption process for the separation of a gaseous mixture, and in particular to a process where a gaseous mixture, under pressure, is conducted over an adsorbent capable of adsorbing at least one component of the gaseous mixture selectively over the other components, and in this way a gaseous stream enriched in one or several other components is recovered at the outlet. The pressure above the adsorbent is then lowered for regenerating purposes so as to liberate a gaseous stream enriched in adsorbed component. The pressure above the adsorbent is then increased and the cycle is repeated. This invention is especially suitable in processes where the adsorbent is a carbon molecular sieve.
Carbon molecular sieves have the particular advantage over other molecular sieves, such as zeolitic sieves, that they are not adversely affected by the presence of water and CO.sub.2 in the mixture to be treated. In comparison, other sieves are provided with upstream adsorbents, such as, for example, silica gel or activated carbon to free the gaseous mixture from water and CO.sub.2, which impurities occur in most technical and natural gaseous mixtures. With carbon molecular sieves, the desorption step results in the removal of these undesirable components along with the other adsorbed components.
A conventional process of this type has been described in DOS [German Unexamined Laid-Open Application] No. 2,441,447 (U.K. Pat. No. 1,480,866). In this process, a nitrogen-enriched stream is obtained by passing air under pressure over the carbon molecular sieve and withdrawing the nitrogen-enriched gas at the outlet end of the adsorber. Once the oxygen content of the discharged gas has reached a specific, previously determined maximum value, the air feed to this adsorber is interrupted, and the gas present in the adsorbent and in the interstices, having an oxygen content higher than that of air, is removed by suction with the aid of a vacuum pump; in this connection, it is desirable to lower the pressure down to at least 100 torr [mm Hg], and most preferably down to 20-70 torr.
To repressurize a regenerated adsorber to the adsorption pressure, the evacuated adsorber, in the conventional process, is first connected to a loaded adsorber and brought into pressure equalization therewith. After pressure equalization has been accomplished, the pressure in the desorbed adsorber is elevated to the adsorption pressure by introducing another gas with the aid of a pump.
In said conventional process, the duration of an adsorption cycle ranges from 40 to 80 seconds; the duration of the pressure equalizing step is between 2 and 3 seconds, and the duration of pressure increase until the final pressure has been reached is in the range from 4 to 6 seconds.
Even prior to said conventional process, the observation had been made that during the separation of air into its components, after desorption and after pressure buildup, the first product nitrogen fraction discharged from an adsorber contains substantially more oxygen than would actually have been expected from a freshly regenerated adsorber. To counteract this undesirable effect and to avoid an initial O.sub.2 surge, the pressure buildup to the adsorption pressure is effected in the conventional process with a portion of the residual gas discharged from another adsorber, i.e., a gas having an N.sub.2 content higher than that of air.
However, it has been found that even this measure cannot control the relatively high initial oxygen concentration to the desired extent. Besides, this procedure requires a second absorber pair connected downstream of the system.